This invention relates to phosphorus-vanadium-oxygen catalysts, and particularly to such catalysts containing tetravalent vanadium. The invention is exemplified by phosphorus-vanadium-oxygen catalysts useful for the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons, and especially for producing maleic anhydride from saturated hydrocarbons, such as butane.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
The prior art teaches that vanadium catalysts are well suited to the production of maleic anhydride from hydrocarbons, usually unsaturated hydrocarbons, and the prior art further teaches that phosphorus-vanadium-oxygen catalysts can be prepared in a number of ways. For example, these catalysts can be prepared by precipitating the vanadium and phosphorus compounds either with or without a carrier from a colloidal dispersion of the ingredients in an inert liquid, and thereafter calcining the precipitate. Catalysts can also be prepared by dissolving vanadium and phosphorus compounds in a common solvent and thereafter depositing the resulting phosphorus-vanadium-oxygen compound from solution on a carrier.
Many prior art procedures for the preparation of vanadium catalysts teach that it is preferable to reduce the vanadium in solution to the tetravalent state. Hence, the prior art teaches that vanadium compounds can be contacted with a reducing acid such as hydrochloric acid or oxalic acid, and then heated until the vanadium is reduced to a valence state of less than five before the compounds are subsequently recovered and used as catalysts.
Although the prior art procedures provide acceptable catalysts, there are attendant processing difficulties with such procedures. As an example, when oxalic acid is used to provide tetravalent vanadium in a phosphorus-vanadium-oxygen catalyst, copious quantities of undesirable oxalate decomposition products are produced, and such oxalate decomposition products are frequently difficult to dispose of by conventional processing equipment. On the other hand, when hydrochloric acid is used to provide such tetravalent vanadium, chlorine and hydrochloric acid are evolved during the heating steps, and can be highly corrosive to the processing equipment used to prepare the catalyst.
These and other disadvantages of the prior art are overcome by the present process for preparing vanadium-containing catalysts which involves the use of a phosphonate compound to provide tetravalent vanadium. It was surprising that phosphonate compounds, which are generally considered to contain pentavalent phosphorus based on their structural formulae, could be used to provide tetravalent vanadium from pentavalent vanadium under conditions that form a phosphorus-vanadium-oxygen catalyst precursor.